Make Version class constructor constant one

[ycherniavskyi]

It will be great to make Version class constructor a constant one, because this will allow using the const Version instance as a default value of an optional parameter.

Also, I see such change is fully backward compatible.

[ycherniavskyi]

For sure, I could make a PR if it is necessary. But this improvement is only about adding const before the Version constructor.

[ycherniavskyi]

Hmm, it seems everything not such easy as I expected :)

lib/version.dart:38:3: Error: A const constructor can't have a body.